This sample case mimics the conditions of the \(M_j=1.086\)  underexpanded jet analysed in [1]}. Due to experimental limitations, no time-resolved data could be obtained in the cited work (as is often the case in shock-containing jets), hindering the identification of wavepackets at off-resonance frequencies. In this previous case, proper orthogonal decomposition (POD) was applied to particle-image velocimetry data of the screeching jet, which allowed for the extraction of the most energetic coherent structure associated with the resonance process. This resonance process leads to the appearance of strongly amplified upstream waves, leading to a flow structure whose main features, such as the strong modulation around the shear-layer, are captured by the absolute instability analysis [2]}. As mentioned earlier, the presence of both upstream- and downstream-travelling waves in the flow at the resonance condition prohibits the application of PFE [3]}, and the resulting modulated wavepacket close to the screech frequency can only be analysed as if there was no screech. With that in mind, the structure of the wavepacket for \(St=0.7\)  also possesses a similar modulation around the centreline, suggesting that some of the features of the modulated wavepacket are preserved in the resonance condition. As mentioned earlier, the structures of the main modulation components (figure REF (f,h)) are also very similar to the wave-like structures highlighted in [1]} for \(St=0.7\) ; this may be due to the presence of both downstream- and upstream-travelling neutral modes in the spatial march, or simply due to the difficulty in separating the structure of the modulation from the different waves supported by the flow.
